The invention relates to improvements in hinge joints in general, and more particularly to improvements in hinge joints which can be used to permit pivotal movements of selected parts of seats relative to each other as well as to hold such parts in selected positions when the adjustment is completed.
It is known to provide the seats in motor vehicles (particularly the driver's seat and the seat next to the driver's seat) with hinge joints which enable the user of the seat or another person to change the inclination of the back rest with reference to the body-supporting or body-carrying part of the seat. The hinge joint includes a first section or leaf which is affixed to the body-supporting part, a second section or leaf which is affixed to the back rest, a shaft which acts as a pintle and articulately connects the second section to the first section, and a locking unit which can hold the second section in a selected angular position so that the inclination of the back rest cannot be changed in response to direct application thereto of a force which urges the second section to pivot with reference to the first section. In many instances, one of the sections has an internal gear which mates with a spur gear of the other section, and the number of teeth in the internal gear exceeds the number of teeth in the spur gear. The shaft of the hinge joint has an eccentric portion which acts as a planet carrier for the spur gear. Such transmissions are known as tumble or wobble transmissions and they produce a self-locking action, in part between the mating teeth of the gears and in part between the spur gear and the eccentric portion of the shaft. The reliability of self-locking action is affected if it is desired to reduce the resistance which the shaft offers to rotation by a hand wheel or the like, particularly by inserting a needle bearing or another antifriction bearing between the eccentric portion of the shaft and the part which surrounds such eccentric portion. When the vehicle is in motion, the back rest of the seat is subjected to continuously varying stresses, partly due to the weight of the person occupying the seat and partly due to unevennesses of the road surface. Thus, the transmission is under constant stress which, in time, leads to at least some changes in the inclination of the back rest, namely in a direction to increase the inclination and to possibly cause discomfort to the occupant. This necessitates repeated adjustments of the inclination of the back rest.
German Auslegeschrift No. 24 46 181 discloses a hinge joint wherein a hand wheel is used to rotate the shaft, and a safety locking device is installed between the hand wheel and the shaft to prevent unintentional angular displacements of the section which carries the back rest. The locking device employs a so-called wrap spring clutch of the type disclosed, for example, in German Offenlegungsschrift No. 1 580 017. A drawback of a wrap spring clutch is that its convolutions occupy a substantial amount of space in the axial direction of the shaft. Therefore, such hinge joint reduces the effective width of the seat and affects the comfort of the occupant.